1. Field of the Invention
This invention concerns detachable mooring systems for loading and offloading liquid petroleum product oil tankers, floating storage (FSO) vessels, floating production storage and offloading (FPSO) systems, floating vessels for natural gas offloading (for example, cryogenic liquefied natural gas (LNG) regas import terminals), and LNG transport vessels.
2. Description of the Prior Art
Numerous patents are known that pertain to disconnectable mooring systems, many of which provide a submerged buoy that can be detachably released from a floating vessel. For example, U.S. Pat. No. 5,651,708 issued to Borseth shows a detachable buoy with a geostationary part. The Borseth buoy has an outer body that is received in a recess in the bottom of the vessel, where the outer body is fixed to the vessel by locking wedges. Four other notable types of detachable mooring systems are known and are illustrated in FIGS. 1 to 4.
FIGS. 1A and 1B illustrate a disconnectable mooring system of a design of FMC Technologies and as illustrated by U.S. Pat. No. 5,240,446. The mooring system includes two basic parts—a geostationary buoy (61) that is detachably connectable to a turret assembly (53) that is disposed in the floating vessel. The buoy (61) is moored to the seabed by a number of anchor legs (63) that are connected to the buoy at anchor leg connectors (62), such that the buoy is generally geostationary.
The vessel (52) carries a turret assembly (53), which is revolvably disposed within the vessel hull and which opens to the sea near the keel elevation. The turret (53) includes a vertical turret shaft (59) and is supported by an upper axial bearing (57) and a lower radial bearing (58). The turret and bearings remain on the vessel when the buoy is disconnected therefrom. The lower end of the turret shaft (59) is equipped with a structural connector (60) that is designed and arranged to disconnectably mate with a connector hub (66) located at the upper surface of the buoy (61). Rubber fenders (64) are provided on the buoy to cushion the mooring process, and a water seal (67) is provided to maintain watertight integrity of the turret compartment in the vessel.
The turret mooring arrangement of FIGS. 1A and 1B provides a fluid flow path between a subsea well or component and the vessel when the vessel is moored to the buoy. The fluid transfer system (FTS) (54) includes a flexible conductor (68) spanning the distance between the seabed and the buoy (61), a lower conductor pipe (56a) that is geostationary and in fluid communication with the flexible conductor, and an upper conductor pipe (56b), which is fixed to the vessel and in fluid communication with the lower conductor pipe (56a) via a fluid swivel (55).
When the buoy (61) is completely separated from the vessel (52), the buoy (61) is designed and arranged to sink to a neutrally buoyant position about 36 meters below sea level. As shown in FIG. 1B, the vessel is moored to the buoy by first recovering the submerged buoy upwards to the structural connector (60) by heaving in a retrieval line (65) with a winch system (not shown). The structural connector (60) is then locked in engagement with the connector hub (66), fixing the turret with the geostationary buoy and mooring the vessel (52) to the seabed. The vessel can freely weathervane about the geostationary turret in response to wind, waves and currents.
FIGS. 2A and 2B show a later version of a disconnectable turret mooring arrangement (71) design of FMC Technologies. The turret mooring arrangement (71) of FIGS. 2A and 2B is substantially similar to the turret mooring arrangement (51) of FIGS. 1A and 1B. For example, the buoy (81) is moored to the seabed by a number of anchor legs (83) that are connected to the buoy at anchor leg connectors (82), such that the buoy is generally geostationary. The vessel (72) carries a turret assembly (73), which is revolvably disposed within the vessel hull and which opens to the sea near the keel. The turret assembly (73) includes a vertical turret shaft (79) and is supported by an upper axial bearing (77) and a lower radial bearing (78). The turret and bearings remain on the vessel when the buoy is disconnected. The lower end of the turret shaft (79) is equipped with a structural connector (80) that is designed and arranged to disconnectably mate with a connector hub (86) disposed at the upper surface of the buoy (81). A water seal (87) is provided to maintain watertight integrity of the turret compartment in the vessel. The fluid transfer system (FTS) (74) includes a flexible conductor (88) between the seabed and the buoy (81), a lower geostationary conductor pipe (76b) in fluid communication with the flexible conductor, and an upper conductor pipe (76a), fixed to the vessel and in fluid communication with the lower conductor pipe (76b) via a fluid swivel (75). When the buoy (81) is separated from the vessel (72), the buoy (81) is designed and arranged to sink to a neutrally buoyant position about 36 meters below sea level. A retrieval line (85) is provided for heaving the buoy to the vessel.
However, unlike the turret mooring arrangement of FIGS. 1A and 1B, where the buoy (61) abuts the keel of the moored vessel (52), in the arrangement of FIGS. 2A and 2B, the upper part of a buoy (81) is cone shaped and is brought into a cone shaped buoy receiving space (89). The structural connector (80) fastens the buoy (81) to the turret shaft (79). The turret shaft (79) is rotatively connected to the vessel (72) by the upper bearing (77). The skirt 90 is rotatively coupled to the lower bearing (78). This system typically is used when several large fluid conductors (88) are required.
FIGS. 3A and 3B generally describe a subsurface buoy mooring system (101) such as that shown by Svensen in U.S. Pat. No. 4,892,495. A cone-shaped buoy (103) is rotatably received into a receptacle (108) formed in the vessel hull (111) and is secured inside a complementary turret receptacle (104) by latches (105). A radial bearing (106) and a vertically-oriented axial bearing (107) support turret (102). The axial bearing (107) abuts a bearing support surface (110). When the buoy (103) is disconnected from the vessel, the turret and the bearings remain on the vessel. The buoy (103) is moored to the seabed by a number of anchor legs (109) such that it is essentially geostationary. For simplicity, the fluid transfer system is not illustrated.
FIGS. 4A and 4B illustrate a type of mooring system (121) design of Advanced Production Loading (APL) AS of Norway and described in U.S. Pat. No. 5,468,166, among others. A buoy assembly (124) includes a buoy (128), upper and lower bearings (126, 127), and a turret (125) that is rotatably supported by the bearings. The cone-shaped buoy (128) is non-rotatably secured into a complementary receptacle (137) formed in the vessel hull (122) by latches (134) that engage a groove (135) formed in the buoy.
The fluid transfer system (FTS) includes a flexible conductor (133) spanning the distance between the seabed and the buoy (128), a lower conductor pipe (132) that is geostationary and in fluid communication with the flexible conductor, and an upper conductor pipe (136), which is fixed to the vessel and in fluid communication with the lower conductor pipe (132) via a fluid swivel (123).
However, the buoy (128) is not geostationary; the buoy is attached to and rotates with the vessel hull (122) while the turret (125) remains geostationary. When the buoy assembly (124) is disconnected from the vessel (122), the bearings and the turret remain on the buoy. The lower end of the turret (125) forms a chain table or anchor leg frame (129) with anchor leg connectors (131). A number of anchor legs (130) connect the turret to the seabed so that the turret (125) is essentially geostationary. In this design the entire anchor leg system weight and loads are supported by the axial bearing (126). Because the APL buoy (128) is secured directly to the vessel (122), its buoyancy does not serve to reduce vertical bearing loads.
Most mooring systems are “turret” systems of one form or another which are familiar to those skilled in the art. Turrets are generally large and expensive structures that usually include large diameter upper and lower bearings. Many prior art disconnectable mooring systems also require a large (approximately 10 meters diameter or larger) cone shaped opening in the vessel bottom. Such structure mandates expensive vessel construction. Because there is a continuing requirement for lowering the cost of major components on floating production systems and loading/offloading cargo vessels, reduction of large, expensive mooring structures is advantageous. Furthermore, large openings in the vessel hull to accommodate mooring buoys cause significant drag and energy losses on those disconnectable cargo vessels when they are sailing long distances. As newer and larger high speed LNG carrier/regas vessels tend to have a narrow flat bottom near the bow at the optimum location for a buoy connection, a large hull opening is less desirable in these applications.
3. Identification of Objects of the Invention
A primary object of the invention is to provide a mooring buoy that remains geostationary with only an inner ring of a bearing mounted on the buoy that can be disconnectably connected to the ship.
Another primary object of this invention is to provide a detachable mooring system in which a bearing can be installed in or on the buoy that has a large radial mooring load capacity due to its unique arrangement. Detachable moorings having larger load capacity are desirable because hydrocarbon production and import/export terminals are moving into more hostile environments.
Another object of the invention is to provide a mooring system that requires a significantly smaller opening in the vessel with the capability to plug the opening so a virtually smooth ship bottom is achieved at the buoy connection point.
Another object of the invention is to provide an improved disconnectable mooring system that eliminates the need for the turret component of prior loading and offloading liquid petroleum product oil tankers, floating storage (FSO) vessels, floating production storage and offloading (FPSO) systems, floating vessels for natural gas offloading, and LNG transport vessels, thereby resulting in significant cost reductions.
Another object of the invention is to provide an improved detachable mooring system that can be released and recovered in high sea states and harsh conditions due to the arrangement of buoy to ship interface equipment.
Another object of the invention is to provide an adaptation of the invention that achieves the inherent cost and functional advantages of the new arrangement for mooring a vessel permanently installed at an offshore location.